The present invention relates to a method and apparatus for electro deposition of metal.
There are various processes and apparatus for electro-refining or electro-winning metal.
One particularly successful process for electro-depositing of copper for example is the so-called ISA PROCESS in which copper is deposited on a stainless steel cathode mother plate. The electrolytically deposited copper is then stripped from the cathode by first flexing the cathode to cause at least a part of the copper deposit to separate from the cathode and then wedge stripping or gas blasting the remainder of the copper from the cathode.
In the ISA PROCESS the bottom edge of the cathode mother plate is generally covered with a release compound such as wax or a plastic edge strip to prevent deposition of copper thereon. This allows for removal of the electro-deposited copper as substantially equivalent separate sheets from both sides of the cathode plate. Such waxing of the cathode sheet, however, is time consuming and there is added cost both for applying the wax and for recovering the wax from the stripping process and associated housekeeping.
To avoid these difficulties, some electro-refining/electro-winning operations use a so-called enveloped cathode process. In such a process the lower edge of the cathode sheet is not waxed and the electro-deposited metal is allowed to grow on both sides of the sheet and around the bottom edge of the cathode mother plate.
Removal of the electrolytically deposited envelope of metal is then accomplished by flexing the cathode and pulling back the metal from both sides of the sheet so that it forms a V. The cathode mother plate is then removed from between the electrolytically deposited envelope of metal, the envelope is then closed and rotated from its vertical position to a horizontal position and transported to a stacking/bundling station.
Not only does such a removal process require complex apparatus for opening the metal envelope, removing the cathode mother plate prior to closing of the envelope and rotating the envelope from the vertical to the horizontal position for stacking, such an arrangement is time consuming and generally not as quick as the ISA PROCESS stripping step.
In conjunction with others, the applicant has recently developed a new process in which an envelope of metal is formed on a stainless steel cathode mother plate and then stripped into two separate sheets. This process is subject of co-pending International Patent Application No. PCT/FI99/00979. By way of summary, this process will now be described with reference to FIGS. 1A-2D.
The initial step in stripping an electrolytically deposited metal envelope from its cathode mother sheet is to at least partially separate either side of the deposited envelope from the cathode sheet. In this regard, reference is made to FIGS. 1A-1D. The enveloped cathode comprises cathode sheets 20 and 30 deposited on the cathode mother sheet 10 and joined along the lower edge thereof by a frangible portion 40. The cathode mother sheet is firstly flexed to provide separation of at least the upper end portion 50 of the sheets 20, 30.
The partially separated envelope as shown in FIG. 1D is then subjected to a stripping operation as shown in FIGS. 2A-2D. The partially separate sheets 20 and 30 are positioned in a stripping apparatus on rollers or conveyor belt 50. The apparatus includes a wedge stripper or air blaster 130. These wedge strippers 130 enter the gap between sheets 20, 30 and cathode mother sheet 10. The wedge strippers 130 essentially separate the entire sheet portions 20 and 30 of the electrodeposited envelope from the cathode mother sheet 10. The sheets 20 and 30, however, are still held together by the frangible portion 40 extending along the bottom edge of the cathode sheet 10 as shown in FIG. 2B. To effect full separation of the electrodeposited metal envelope from the cathode mother sheet 10 into separate substantially equivalent sheets 20 and 30 is held by grippers 25, 35 and rotated about the frangible portion 40 from the substantial vertical position shown in FIG. 2B to the substantially horizontal position shown in FIG. 2C. This rotation separates the deposited metal from the cathode into two substantially equivalent sheets. In many cases, a single rotation of the sheets 20, 30 from the vertical to the horizontal is all that is required to separate the sheets. This separation of the sheets 20 and 20 from each other as well as the cathode mother plate may be confirmed by the grippers 25, 35 as follows. The grippers which still hold the sheet 20, 30 in horizontal position shown in FIG. 2C, are adapted to pull the respective sheets slightly outward as shown in FIG. 2D. If the sheets, 20, 30 move outwardly in unison with the grippers, separation of the sheets 20, 30 is confirmed. If, however, the force to move the grippers outward is too great or simply the grippers do not move this indicates that the frangible portion 40 has not in fact separated the sheets 20, 30 and accordingly further rotation (as shown in FIG. 2C) of the sheets may be required.
If further manipulation/rotation of sheets 20, 30 is required, the apparatus using grippers 25 and 35 rotates sheets 20 and 30 upwardly and downwardly until the aforementioned confirmation of separation of the sheets is effected.
In a preferred embodiment, cathode sheet 10 may be lifted upwardly in the stripping apparatus to provide more clearance between it and the sheets 20, 30 and frangible portion 40 since manipulation of the sheets 20 and 30 may cause contact between at least the frangible portion 40 and the cathode sheet 10.
Once the cathode sheets 20 and 30 are separated into substantially equivalent separate sheets, it is a simple matter to transport the sheets out of the apparatus for stacking and subsequent treatment.
The growth of this deposited metal envelope, however, is complex and the applicant has found that under certain process conditions it may be difficult to separate the electro-deposited envelope into two separate sheets. This is particularly true if, for any reason, power supplied to the electrolytic bath is interrupted for any substantial length of time. If this occurs, the metal sheets require rotating or flapping several times to effect separation.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative thereto.
In a first aspect, the present invention provides a method for electro depositing metal on a cathode in an electrolytic cell, said method comprising applying an electric potential to the cell to deposit an envelope of metal on said cathode, said envelope including two substantially equivalent sheets on either side of said cathode joined along at least one edge portion by a frangible region, the metal being removable from the cathode by rotation of the respective sheets about the frangible region,
wherein the direction and quantity of current in the electrolytic cell is monitored such that as current flow approaches or reaches a predetermined value and/or the direction of current flow changes, an auxiliary power supply applies an auxiliary potential to the cell at a level sufficient to maintain a predetermined direction and quantity of current flow in the cell.
Not wishing to be bound by any particular theory, the present applicant has found that power interruption for any considerable period of time (ie. one hour or more) in the cell can result in xe2x80x9claminationxe2x80x9d of the metal in the area of the frangible region. To explain, if power is supplied to the electrolytic cell resulting in a xe2x80x9cforwardxe2x80x9d current, deposition of metal from the anode to the cathode is maintained and the metal is deposited in a controlled orderly fashion.
On the other hand, if power is interrupted and later recommenced, the orientation of metal deposition appears to alter. It is believed this is due to the metal treating the exterior surface of the already deposited metal as a fresh surface on which to deposit. Accordingly there may be several xe2x80x9cdirectionalxe2x80x9d changes of deposited metal crystals in the area of the frangible region if power is interrupted on more than one occasion. This results in laminates of different crystal orientations appearing in the metal.
The boundary layers between such laminations can act as fault lines resulting in unpredictable and non-uniform separation of the deposited envelope of metal into two separate sheets. By maintaining a predetermined direction and quantity of current flow in the cell, the metal crystals deposit in a uniform and consistent matter thereby avoiding such laminates of different crystal orientations.
The auxiliary power supply may be activated during the entire period of metal growth on the cathode such that power never drops to below a predetermined level resulting in zero or backward current. Alternatively, the auxiliary power may be activated only when main power supply is reduced or fails.
In a further aspect, the present invention provides a method of providing power to an electrolytic cell to deposit metal on a cathode comprising providing a main power supply and an auxiliary power supply to the cell, the auxiliary power supply being sufficient to maintain a predetermined direction and quantity of current flow in the cell when activated.
In yet a further aspect, the present invention provides an apparatus for maintaining electro-deposition of metal on a cathode in an electrolytic cell, said electrolytic cell comprising a metal anode, a cathode, an electrolytic bath and a main power supply to apply an electric potential across the anode and cathode resulting a forward current and deposition of metal from said anode to said cathode,
said apparatus including an auxiliary power supply adapted for connection to the cell such that in cases of mains power supply reduction or failure, said auxiliary power supply maintains a predetermined direction and quantity of current flow in the cell.
Unless the context clearly requires otherwise, throughout the description and the claims, the words xe2x80x98comprisexe2x80x99, xe2x80x98comprisingxe2x80x99, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of xe2x80x9cincluding, but not limited toxe2x80x9d.